Vacuum tube voltmeter



July 25, 1950 H. LAMPORT ErAL VACUUM TUBE VOILTMETER Filed March 2'7, 194'? INVEN TOR.

HAROLD LAMPORT MARVIN G.SCHORR ATTORNEY Patented July 25, 1950 UNITED STATES PATENT VACUUM TUBE VOLTMETER Harold Lamport, New, Haven, and Marvin G.

Schorr, HamdemConn.

Application March 27, 1947, Serial N 0. 737,654

1 Claim.

This invention relates to vacuum tube voltmeters and more particularly to a portable millivoltmeter for the measurement of bioelectric potentials.

Millivoltmeters in use heretofore, have utilized large power type tubes having a high grid-toplate transconductance. These tubes were operated at reduced voltages to achieve both stable and low grid current. Where portability is of prime importance, these millivoltmeters have several serious disadvantages. Because of their large current requirements, a heavy duty B-battery is needed for the plate circuit and a storage battery is necessary for the filaments. Despite the use of these large batteries, the high current drain causes considerable instability and a balanced bridge circuit is needed to eliminate supply variations. In achieving the requisite stability, the bridge doubles the current requirements while halving the sensitivity of the instrument as compared to that of a single tube. While less sensitive tubes of lower transconductance designed for dry cell operation may be chosen, they also must employ the balanced circuit with its attendant disadvantages.

An object of the present invention therefore, is to provide a light, compact, portable meter particularly suited for the measurement of bioelectric potentials in which the above disadvantages are eliminated.

A further object of this invention is to provide a meter with the sensitivity and stability of nonportable meters using large power type tubes.

Other objects and advantages will become apparent from the following specification, wherein the single figure represents the schematic diagram of a preferred embodiment of the invention.

Referring now to the drawing, tube 23 is a triode amplifier with the input being applied to the control grid and the output taken from the anode in the usual fashion. The voltage to be measured is applied across a resistor II which, because of the biological applications of this invention, should be at least 10 megohms. The

amplified output appears in the anode circuit the zero-setting control, and resistor 26 is the Vernier adjustment for galvanometer 3U.

An unbypassed variable resistor 2| in the filament circuit provides cathode bias thereby obviating the need of a bias cell. Since resistor 2| is unbypassed, negative feed-back is obtained, thereby increasing the stability of the unit. Battery 22 provides the required filament voltage. Variable resistor H is used to adjust the filament current to the required rating. Milliammeter l9, which can be inserted in the filament circuit, is used in conjunction with resistor ll to set the magnitude of the filament current.

A calibration circuit comprising fixed resistors l2 and I4, rheostat I3, and battery I5 is utilized to develop a desired voltage drop between grid and cathode to be used for calibration. Rheostat I3 is adjusted to secure the proper value of voltage drop across resistor l4, whose value is known. Resistor I8 is a dummy resistor to be in-'- serted into the filament circuit to maintain the proper filament current when the calibration circuit is in use. The resistance of resistor I8 is equal to the resistance of meter I9.

Various adjustments are made before measurements are taken. The closing of double-pole single-throw switch 24 completes the filament circuit. When double-pole double-throw switch I6 is in position .w-a, milliammeter i9 is in the fila ment circuit and variable resistor H is adjusted to secure the desired amount of filament current, as indicated on the milliammeter.

To operate the calibration circuit, switch It is shifted to position bb. This inserts milliammeter IS in the calibration circuit and replaces it with resistor l8 in the filament circuit, thus preventing any variation in the filament current while the unit is being calibrated. Variable resistor I3 is adjusted to secure the desired voltage drop across resistor M as indicated bythe reading on milliammeter I9. The bias developed between the grid and the cathode is the sum of the voltages across resistors I4, 2| and 20. Resistor 2| also controls the amount of the negative feedback which is developed due to the fact that resistors 20 and 2| are unbypassed. For proper operation, the grid must be biased to the value at which no grid current flows. This point is reached if the deflection of galvanometer 30 remains unchanged when switch I0 is shifted from position b to position 0. If a deflection occurs, resistors I3 and 2| must be reset to eliminate the grid current that still remains.

- With the calibration circuit properly adjusted, resistor 21, the sensitivity control, is adjusted to 3 secure the desired deflection range for galvanometer 30.

Closing switch 24 also completes the balance out circuit for galvanometer 30, and current flows from battery 28 through galvanometer 30 and through resistors 26, 25 and 29. This current flows through galvanometer 30 in a direction opposite to that of the anode current and resistor 25 is adjusted until the magnitude of both currents is equal, at which point there will be no indication of a current flow through galvanometer 30. Resistor 26 is the Vernier adjustment for zero control 25.

In one embodiment of this invention, with an input resistance of at least 10 megohms, the grid current must be less than 10- amperes with a 1100 millivolt range of galvanometer 30 to achieve a sensitivity of one millivolt per scale division or approximately .04 microampere per scale division. The following values of circuit components'are suggested for this embodiment:

Resistor I l, 10 megohm-carbon resistor Resistor 1| 2, lOOohm wire Wound resistor Resistor I3, 100 ohm Wire wound variable resistor Resistor l4, 2 ohm precision wire Wound resistor Resistor I11, 50 ohm Wire wound variable resistor Resistor :18, resistor equal to resistance of milli- .ammeter 19 Resistor 20,5000 ohm wire wound resistor Resistor 2i 5000 ohm wire wound variable rezsistor Resistor 25, 5000 ohm wire wound variable resister Resistor v20, 500 ohm wire wound variable resistor Resistor 21,20,000 ohm wire wound variable resister Resistor 29, 7000 .ohmwire wound-resistor Milliammeter 10, -15 milliammeter Battery 1-5, 1.5 volts Battery '22, 1.5 volts Battery 28, 3 volts Battery Bl, 10.5 volts Tube 23, in this embodiment is a type VW-32 miniature tube manufactured by Victoreen Instrument 00., Inc., Cleveland, Ohio. The filament draws less than 15 milliamperes from a single 1.5 volt cell. The anode current is about 250 microamperes with a 10.5 volt B-battery. Because of these low current requirements, flashlight cells for the anode supply and a single Number 6 type dry cell for the filament can be used in an unbalanced circuit Without significant drift. At these low operating voltages, residual grid current is easily eliminated, provided no light falls on'the tube. Where the full range of :IOOmillivolts is required on a single scale, without the use of a reversing switch, a galvanometer with a scale length or '100 millivolts adjusted to two millivolts per millimeter is preferred. While the rated filament current for the tube is .15 milliamperes most satisfactory operation is found at 14 milliamperes. Resistor I! adjusts the filament current to this value as read on milliammeter 19. When switch 16 is in position bb milliammeter i9 is in the calibration circuit. Rheostat i3 is adjusted so that milliamperes flow through the two ohm resistor l4 thus providingthe 20 millivolt calibrating potential. Rheostat 21 adjusts the galvanometer sensitivity so that the calibrating voltage causes the desired deflection. Rheostat 2| isset for zero grid current, as previously explained, so that. the quiescentanode current line 4 dication on galvanometer 30 can be balancedout by adjusting resistors 25 and 26.

Ordinarily, the setting of resistor 2| is made only once, after the tube has been aged by about 10 hours of continuous operation. When grid current is eliminated at zero input potential, it remains substantially nil throughout the range of the instrument. With the component values stated above it was found that after this instrument had been used for more than hours in the measurement of direct current potential dif-' ferences occurring on the skin of human subjects, the initial drift Was under one millivolt per hour, increasing slightly as the filament battery were down. However, when filament current is maintained constant by adjusting resistor H, the drift is eliminated, indicating that the filament battery is its source.

The complete instrument, including batteries, can be incorporated in a six inch by six inch by six inch metal cabinet which also acts as a shielding.

While what has *been described is a preferred embodiment of our invention, it is not intended to limit our invention to the embodiment herein described but to include all modifications thereof that are apparent to one skilled in the art.

We claim:

In a vacuum tube voltmeter, a variable bias circuit comprising in combination a vacuumtube having at least an anode, a grid and a cathode, a first direct-current source having a negative and a positive terminal, said source being connected between said anode and cathode, a cathode heater circuit, a reference potential, a first resistor connected between said grid and said reference potential, a calibration circuitcomprising a second resistor connected between the negative terminal of said source and said reference po-jtential, a rheosta-t connected to said reference potential, a milliammeter having a positive ter minal and a negative terminal, the positive terminal of said milliammeter being connected to the other end of said rheostat, a second direct current source having a negative and a positive terminal, the negative terminal of said source being connected to the negative terminal of said milliammeter and its positive terminal connected to the negative terminal of said first sourcee. third resistor having a resistance equal to that of said. milliammeter, and switching means to alternately insert said milliammeter in series with said cathode heater circuit or said calibration circuit and to insert said third resistor into the heater .circuit when said .milliammeter is switched into said calibration circuit. .1;

HAROLD LAMPORT. MARVIN G. SCHO-RR. .2;

REFERENCES siren The following references are of recordin file of this patent:

UNITED STATES PATENTS the M OTHER REFERENCES Electronics, Improved Vacuum-Tube V6 15 meters, by J. T. McCarthy, July l9 l5f-pages 137-139. 

